This invention relates to the field of light-emitting devices.
Over the past decade, organic light emitting diodes (OLEDs) have shown increasing promise as inexpensive and efficient light sources. The devices are inexpensive to manufacture as a result of the device layers being composed of amorphous organic materials. Consequently, these layers can be inexpensively deposited by, for example, low vacuum thermal evaporation, spin casting, and ink-jet techniques. In contrast, inorganic light emitting diodes (LEDs) are composed of crystalline materials which require expensive deposition equipment, such as metal organic chemical vapor deposition and molecular beam epitaxy, and can only be deposited on specific substrates. The net result of this difference is that OLED-based multicolor arrays composed of hundreds of thousands (to millions) of pixels are being made routinely by many manufacturers worldwide largely for display applications.
With regard to the efficiency of OLED devices, their external quantum efficiencies are typically in the 1 to 3% range, in spite of their internal quantum efficiencies being as high as 80 to 90%. Part of this discrepancy is the result of spin statistics, whereby for non-phosphorescent materials three-quarters of the excitons are non-emitting triplets. However, a larger factor contributing to the difference in quantum efficiencies is the result of light-piping, where simple analysis (N. C. Greenham et al., Adv. Mater. 6, 491 [1994]) based on total internal reflection shows that only xc2xdn2 (n is the index of refraction of the device layers) of the light generated in planar devices actually exits the device. For OLED devices n is on the order of 1.9; therefore, only 14% of the generated light escapes out of the top of the device.
Over the past decade a concerted effort has been expended by both inorganic and organic-based LED researchers to find some means for increasing the out-coupling efficiency of LED devices. Bulovic et al. in U.S. Pat. No. 5,834,893 have suggested growing the OLEDs in metallic pits with slanted side walls in order to out-couple the device""s waveguide modes. A number of authors, such as Yamasaki et al., Appi. Phys. Lett. 76, 1243 (2000) and Windisch et al., Appl. Phys. Lett. 74, 2256 (1999) used scattering centers (volumetric or surface, respectively) to enhance the external efficiency. Others have depended on periodically positioned scatterers to enhance the out-coupling efficiency. For example, I. D. W. Samuel et al. in WO 00/70691 employed Bragg gratings to increase the out-coupling, while Erchak et al., Appl. Phys. Lett. 78, 563 (2001), used two-dimensional photonic crystal structures. In each case the extraction efficiency was enhanced; however, at the price of a loss of acutance. In other words, the enhanced out-coupling occurs over a distance larger than the original size of the pixel, which leads to an unwanted increase in the pixel dimensions.
A means for avoiding this acutance loss, while maintaining the enhanced out-coupling, is to employ a resonant cavity LED (RCLED) structure. Such a structure was originally proposed by Cho et al. in U.S. Pat. No. 5,226,053 for inorganic material systems. In their work they obtained approximately a factor of 1.7 increase in external efficiency relative to their LED control sample (E. F. Schubert et al., Appl. Phys. Lett. 60, 921 [1992]). Later, Jacobsen et al. in U.S. Pat. No. 5,804,919 made use of the RCLED concept to enhance the emission of phosphors for display applications. As pointed out by Cho et al. in U.S. Pat. No. 5,226,053, proper operation of the RCLED requires device structures very analogous to those of vertical cavity surface emitting lasers (VCSEL), except for one of the dielectric stack reflectivities being reduced from typical VCSEL values of  greater than 99%. In U.S. Pat. No. 5,804,919 to Jacobsen et al., there is no mention of the importance of lowering one of the dielectric stack reflectivities. In addition, Jacobsen et al. teach one to use bulk active region thicknesses, instead of thinner active regions ( less than 30 nm) surrounded by spacer layers, which is required to get good efficiencies while keeping the unwanted spontaneous emission to a minimum. As a result, even though Jacobsen et al., in U.S. Pat. No. 5,804,919, mention employing organic active regions as the emitters, the structures they discussed are not properly constructed RCLED devices. Furthermore, the only organic emitter Jacobsen et al. mention is the host material Alq [aluminum tris(8-hydroxyquinoline)], which has very poor luminescent properties. As a result, to this date the RCLED concept has not been properly applied to organic emitter applications, where as discussed above it should enable enhanced out-coupling efficiencies, without the concomitant loss of acutance.
It is an object of this invention to provide a vertical cavity light-producing device that produces quasi-laser light with a spectral linewidth selected to improve power conversion efficiency.
These objects are achieved by a vertical cavity light-producing device which, in response to incident external light, produces quasi-laser light with an enlarged spectral linewidth selected to improve power conversion efficiency, comprising:
a) a substrate;
b) a bottom dielectric stack reflective to light over a predetermined range of wavelengths;
c) an organic active region for producing quasi-laser light;
d) a top dielectric stack spaced from the bottom dielectric stack and reflective to light over a predetermined range of wavelengths;
e) the organic active region includes one or more periodic gain region(s) and organic spacer layers disposed on either side of the periodic gain region(s) and arranged so that the periodic gain region(s) is aligned with the antinodes of the device""s standing wave electromagnetic field; and
f) the top or bottom dielectric stack being selected so that it""s peak reflectance is less than 99% and the device""s spectral linewidth is increased but produces an acceptable level of spontaneous emission, thereby resulting in improved power conversion efficiency.
Quite unexpectedly, it has been determined that by proper selection of the reflectance of one of the dielectric stacks, the power conversion efficiency can be substantially increased, while the unwanted spontaneous emission is kept acceptably small. In addition, the radiated light maintains a narrow spectral linewidth and is emitted directionally about the optic axis of the device.
It is an advantage of the present invention to improve the device efficiency operation of a vertical cavity light-producing design by incorporating top and bottom dielectric stack reflectors, having gain regions consisting of organic material, with the gain region(s) placed at the antinodes of the standing wave electromagnetic field of the device. The reflectivities of one of the dielectric stacks is chosen to be high as typical for VCSEL devices, while the opposing dielectric stack""s reflectance is lowered to less that 99%. As a result, the power conversion efficiency is improved, unwanted output due to spontaneous emission remains significantly reduced, while the radiated light maintains a narrow spectral linewidth and is emitted directionally about the optic axis of the device.